1. Field of the Invention
This invention relates generally to the field of electric well logging. More particularly the invention relates to induction logging. Still more particularly the invention relates to a method for determining sonde error which is the signal an induction tool measures when the tool is placed in a zero-conductivity medium.
2. Description of Prior Art
Ideally an induction logging tool should read zero in a zero-conductivity medium. However, conductive material used in the sonde construction, (such as sonde wiring, quadraxes, bulkheads, electrodes for spherical focused logs mounted on the induction sonde, etc.) produce a non-zero sonde error signal. Certain new induction sondes measure not only the real or "R" conductivity signal (the ratio of the signal from the receiver coil array which is one hundred eighty degrees out-of-phase with the transmitter array current), but also the quadrature or "X" conductivity signal (the ratio of the signal from the receiver coil array which is ninety degrees out-of-phase with the transmitter array current.) In the case of the X signal, the unbalanced mutual from the transmitter is also a major contributor to sonde error.
Once the sonde error for a particular logging tool to be used in well logging operations has been determined, it is subtracted from the raw tool response during logging, or in processing raw tool data in a computerized instrumentation unit to correct the raw tool data for the sonde error signal.
Accurate determination of induction sonde errors has historically been a difficult and inexact art. Some field engineers have attempted to determine sonde error while the induction tool is downhole. But such a procedure inherently corrects for borehole signal and shoulder effect at a particular point in the borehole and may produce erroneous measurements at different locations in the borehole. Other operating well-logging field engineers have measured sonde errors with the induction tool dangling from a tall pole or support structure. In other words, the transmitter coil is energized, the voltage from the receiver coil array is measured and the conductivity reading of the tool is determined while it is dangling from the pole and is defined as the apparent error. The major obstacle with this prior art method and others, has been in accurately determining the background signal which is the signal from conductive surroundings about which the measurement is taken. The background signal, usually "estimated" by field engineers, must be subtracted from the apparent sonde error in order to determine the "true" sonde error connection.
There has been no uniform method in field operations for determining background signals. In addition, background signals are subject to change, because background signal depends on whether the earth under the tool during the sonde error determination is wet or dry and upon the presence or absence of metallic objects, such as cars, trucks, buildings, etc., in the vicinity of the tool during the measurement.
Compounding the sonde error determination problem is the requirement of certain new induction logging tools which not only measure the real conductivity signal, but also the quadrature conductivity signal and does so at multiple frequencies. Such a tool is described in U.S. Pat. Nos. 4,471,436, 4,513,376, and 4,467,425, which are assigned to the assignee of this invention. The sonde error of such a tool must be determined for each real and quadrature conductivity component at each operating frequency for which such components are determined.